The spinal column is a highly complex system of bones and connective tissues that provides support for the body and protects the delicate spinal flexible connecting member and nerves. The spinal column includes a series of vertebrae stacked one on top of the other, each vertebral body including an inner or central portion of relatively weak cancellous bone and an outer portion of relatively strong cortical bone. An intervertebral disc is situated between each vertebral body to cushion and dampen compressive forces experienced by the spinal column. A vertebral canal containing the spinal cord and nerves is located posterior to the vertebral bodies. In spite of the complexities, the spine is a highly flexible structure, capable of a high degree of curvature and twist in nearly every direction. For example, the kinematics of the spine normally includes flexion, extension, rotation and lateral bending.
There are many types of spinal column disorders including scoliosis (abnormal lateral curvature of the spine), kyphosis (abnormal forward curvature of the spine, usually in the thoracic spine), excess lordosis (abnormal backward curvature of the spine, usually in the lumbar spine), spondylolisthesis (forward displacement of one vertebra over another, usually in a lumbar or cervical spine) and other disorders caused by abnormalities, disease, or trauma, such as ruptured or slipped discs, degenerative disc disease, fractured vertebra, and the like. Patients that suffer from such conditions usually experience extreme and debilitating pain as well as diminished range of motion and nerve function. These spinal disorders may also threaten the critical elements of the nervous system housed within the spinal column.
One particular spinal fixation technique includes immobilizing portions of the spine of a patient by using connecting elements such as relatively rigid orthopedic spine rods that run generally parallel to the spine. Another technique utilizes less rigid connecting elements to provide a more dynamic stabilization of the affected regions of the spine. One example of such a spinal stabilization system is offered by the assignee of this invention, Zimmer Spine, Inc., as Dynesys® and is disclosed in European Patent No. 669,109, which is hereby incorporated by reference entirely. As used herein, the terms “spinal stabilization system”, “spinal stabilization construct” and similar terms encompass any type of connecting element extending between adjacent vertebrae regardless of its rigidity, flexibility or construction. Installation of such systems may be accomplished, for example, by exposing the spine posterially and fastening hooks, bone screws, or anchors to the pedicles of the appropriate vertebrae. The vertebral anchors may be generally placed in a quantity of two per vertebrae, one at each pedicle on either side of the spine and serve as anchor points for the connecting elements.
Installation of such spinal stabilization constructs with vertebral anchors may thus require a surgeon to prepare an incision aligned with the spine of a patient. The vertebral anchors may then be attached to a number of vertebrae after which the connecting element is located with respect to receiving portions of the vertebral anchors. Fastening of a vertebral anchor may involve inserting a tool through the incision to engage the anchor such that actuation of the tool causes the anchor to engage the pedicle or other suitably chosen part of the vertebra. For example, a top portion of a threaded pedicle screw can be engaged by a tool such that rotation thereof in turn rotates the pedicle screw. Rotation of the pedicle screw then threadably engages the vertebra thereby securely mounting the anchor.
Known installation systems and techniques for spinal stabilization systems may be such that an interface between an instrument and an anchor allows some level of loose rotational and/or axial engagement or “play” between them, which may be undesirable to some users. Some systems also include multiple instruments to allow for engagement and rotation of the anchors and do not allow for top loading of elements of the stabilization system into the anchors. Therefore, systems and related methods to provide installation of a spinal stabilization construct and which address some of the drawbacks of known systems are highly desirable.